A seismic sensor has one or more sensing elements, each designed to measure acceleration in a specific direction (subtracting constant gravitational acceleration) and to be insensitive to acceleration orthogonal to that direction. One class of seismic sensors utilizes a spring, herein called a main spring, which balances a pendulum at an equilibrium position and applies a restoring force on the pendulum towards the equilibrium position when the pendulum is displaced from the equilibrium position.
One class of main spring is characterized by having one end of the spring attached to the pendulum to apply a torque to the pendulum, and having the other end tensioned by an attached length of wire that is in turn attached to a fixed frame. An adjustment mechanism on the fixed frame can be provided that pulls the wire to adjust the tension of the spring once the seismometer is installed in position, to balance the boom at the desired equilibrium position. The use of a flexible wire allows the adjustment mechanism to be positioned at a convenient distance from the end of the spring and allows the adjustment mechanism more degrees of freedom than if the end of the spring was fixed directly to the adjustment mechanism. This arrangement has several disadvantages, including:                the attachment of the wire to the end of the spring usually requires a lumped mass at the attachment point, typically consisting of a screw, washer and nut, and this lumped mass at the end of the spring can give rise to undesirable resonances;        it is difficult to assemble the spring so that the wire is perfectly straight and free of bends or kinks when installed. The bends may creep or relax over the operating life of the seismometer, causing unwanted spurious signals (noise) in the output;        the wire must be thin to be sufficiently flexible, which makes it susceptible to breaking due to mechanical shock in transport or handling; and        the wire is subject to stress concentration at the ends where it is attached and is particularly susceptible to breakage at these points.        
Another class of main springs is characterized by having one end of the spring attached to the pendulum to apply a torque to the pendulum, while having the other end directly attached to a fixed frame. An adjustment mechanism on the fixed frame may be provided that varies the torque applied to the end of the main spring to balance the pendulum at the desired equilibrium position. This arrangement also has disadvantages, including:                it is difficult to make a pure kinematic adjustment mechanism that adjusts the position of the end of a flat spring, and therefore the adjustment mechanism can be a source of spurious transient signals or noise, sometimes also called “pops”; and        with both ends fixed, the spring geometry and therefore tension will change as the length varies due to thermal expansion. This is undesirable as temperature changes can modulate the output signal and be falsely seen as seismic disturbances.        
There is a need to provide seismic sensors having main springs that address at least some of the previously recited disadvantages.